Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a Multiple-Input Multiple-Output (MIMO) communication channel and such systems and/or related techniques are commonly referred to as MIMO systems.
With the assistance of the channel quality measurements for link adaptation and scheduling, advanced multi-antenna i.e. MIMO schemes are supported by 3rd Generation Partnership Project (3GPP) developments within the Long Term Evolution (LTE) project. Precoding is a promising technique among multi-antenna schemes to improve throughput performance. In LTE, there are two basic methods to obtain the precoder, one is codebook-based and the other is non-codebook-based precoding. In the former method, a set of precoder candidates, a codebook, is pre-defined on both eNodeB i.e. base station and user equipment (UE) sides. The UE selects a preferred precoder from the set and feeds back this preferred precoder to the eNodeB in the form of a precoder index. Then eNodeB takes this feedback into consideration when it decides whether to use the suggested precoder or not. The eNodeB also informs the UE via downlink (DL) control channels about the index of the used precoder. For LTE release 8, cell specific reference signals are transmitted in DL. The UE uses them to obtain channel estimates for the eNodeB antenna ports. These channel estimates can be used to determine the Pre-coding Matrix Index (PMI), but also for demodulation given that the user equipment knows the applied precoder matrix so that it can construct an estimate of the effective channel, which is a product of the channel and the precoder matrix. Therefore, certain feedback and feed-forward signalling is necessary.
As mentioned earlier, in addition to the codebook-based beamforming, LTE also supports more general non-codebook-based beamforming. This support is provided with a so called UE specific, or Dedicated Reference Signal (DRS), which are precoded in the same way as the data and hence can be used for channel estimation for demodulation. This is suitable for both FDD and Time-Division Duplexing (TDD) systems. For TDD, short-term precoding based on instantaneous channel knowledge is possible, whereas for FDD one has to resort to more long term precoding based on estimates of channel statistics rather than realizations. If TDD based systems are in place, the base station is not constrained to select precoding vectors from a certain limited set, but can exploit channel reciprocity to adjust the downlink transmission weights from channel estimates. These estimates could at least be partial Channel State Information at the Transmitter (CSIT) obtained via any uplink transmission process such as Sounding Reference Signal (SRS), or control signalling on Physical Uplink Control Channel (PUCCH) or data transmission on the Physical Uplink Shared Channel (PUSCH), and possibly better optimize the transmission as compared to what is possible if only precoders from a codebook can be used. It may be noted that PUCCH and PUSCH contain reference signals for demodulation, typically covering the same bandwidth as the payload part whereas a sounding reference signals is not associated with transmission of any payload but only transmitted to enable uplink channel estimation with configurable bandwidth and periodicities.
In transmission mode 7, a dedicated reference signal precoded in the same way as the data is used to demodulate the data transmission at the UE, and hence facilitates non-codebook based beamforming. Though DRS provides simpler UE receiver, extra functionality needs to be implemented in the user equipment to support demodulation using UE specific reference signals in addition to demodulation using cell specific reference signals. This feature is an optional one, thus not all UEs in LTE systems are expected to support demodulation using user equipment specific reference signals.
Moreover, the DRS occupy resources for each UE that can not be used for data transmission. In parallel to this, Common Reference Signals (CRS, also known as Cell-specific Reference Signals) are always allocated for purposes such as transmission of common control information, mobility measurements and Channel Quality Indicator (CQI) measurements in LTE Releases 8 and 9.
Furthermore, cell-specific reference signals are typically transmitted in all downlink subframes and over the entire bandwidth. This allows the user equipment to perform interpolation and averaging in the time and frequency domain to improve channel estimation. For the DRS, the user equipment is not allowed to do this but must only use the resource elements with reference signals within a single physical resource block pair. Hence, averaging is allowed only up to a bandwidth of 180 KHz and a time duration of 1 ms.
Blind precoding has been proposed, which focuses on precoding matrix estimation at the receiver side for multi-stream transmission. The blind precoding method is very sensitive to the UE movement and the corresponding Doppler spread. It has been shown that the time correlation between wDL (precoder weights determined at downlink receiver, i.e. eNodeB) and wUL (precoder weights determined at uplink receiver, i.e. UE) drops much faster than the correlation between the elements of the channel matrix. This indicates that after an Uplink (UL) and Downlink (DL) delay, the elements of HUL and HDL may still be related very closely, but the wUL and wDL can be largely different. Here, HUL and HDL are channel matrices for DL and UL direction respectively. So, it could only be applicable for the scenario with a slow time-variant channel, where HDL≈HUL.
Previously for Wideband Code Division Multiple Access (WCDMA) and Code Division Multiple Access (CDMA) 2000 systems, downlink beamforming using common pilot signals, similar to CRS in the LTE system, have been studied, where the beams for data transmission are adapted to match the common pilot beam. Furthermore, previous designs for precoding using cell specific reference signals for WCDMA are designed for a single common reference signal and can hence not be used when there are multiple cell specific reference signals.